Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. LEDs have substantially higher light conversion efficiencies than incandescent lamps and longer lifetimes than both types of conventional light sources. In addition, some types of LEDs now have higher conversion efficiencies than fluorescent light sources and still higher conversion efficiencies have been demonstrated in the laboratory. Finally, LEDs require lower voltages than fluorescent lamps, and hence, are better suited for applications in which the light source must be powered from a low-voltage source such as a battery or an internal computer DC power source.
Unfortunately, LEDs produce light in a relatively narrow spectral band. To replace conventional lighting systems, LED-based sources that generate light that appears to be “white” to a human observer are required. In addition to appearing white when viewed directly by user, the light source must also have a good color-rendering index so that the generated spectrum allows the user to view a colored object or scene and see the colors of the objects. The ability to accurately perceive the color of an illuminated scene is referred to as the color-rendering index of the light source. A light source with three narrow spectral bands at the red, green, and blue wavelengths looks white to a person viewing the light source, but has a very poor color-rendering index, since an object that reflects light at wavelengths between the spectral band will have a distorted color presentation. To provide a good color-rendering index, the light source must provide light at all wavelengths.
Phosphor conversion of light generated by an LED can be utilized to provide a white light source that has an adequate color-rendering index. A light source that appears to be white and that has a conversion efficiency comparable to that of fluorescent light sources can be constructed from a blue LED that is covered with a layer of phosphor that converts a portion of the blue light to a broad band of yellow light. Such light sources will be referred to as “phosphor-converted” light sources in the following discussion. If the ratio of blue to yellow light is chosen correctly, the resultant light source appears white to a human observer.
The color-rendering index of such yellow phosphor-converted light sources is significantly less than that provided by an incandescent bulb. In particular, the spectrum from this type of phosphor-converted light source has insufficient intensity in the red portion of the optical spectrum to provide a color rendering index that is as good as that provided by an incandescent bulb. As a result, in some applications, a red LED is added to the yellow-phosphor converted light source to improve the color-rendering index. In addition, the ratio of the red light to the blue light from the blue LED can be adjusted to provide a “warm” white light source in such two LED light sources.
The efficiency of a light source is defined to be the fraction of the electrical energy consumed by the light source that is converted to light in the region of the optical spectrum to which the human eye is sensitive. The overall efficiency of a phosphor-converted light source is determined by the efficiency with which the LED converts electricity to light and the efficiency with which the LED light is down-converted to light in the portion of the spectrum to which the eye is sensitive. When blue light is converted to light at longer wavelengths, the difference in energy between the blue photons and the longer wavelength photons is lost in the form of heat. For a blue to yellow conversion, the loss is a significant fraction of the energy. In addition, the yellow phosphors that are used in many applications also convert a significant fraction of the blue light to wavelengths that are greater than 630 nm. The sensitivity of the human eye to light in this region is small, and hence, this energy is, in effect, wasted.